When updating a circuit, there is first produced one or more new circuit masks to replace or renew the mask or masks in which pattern changes have been made. A new circuit of the same kind as the earlier circuit, but having a new updating status, is produced from the newly manufactured, revised masks and the remaining old masks of the original circuit. When practicing known techniques, the circuit cover is marked with a code which identifies the updating status of the circuit and which is checked prior to installing or fitting the circuit, so as to ensure that a circuit having the correct updating status is fitted. This code check is made ocularly.
Mask programmable read-only-memories are known in connection with integrated circuits. The memory content of these read-only-memories is determined by the configuration or design of one of the pattern layers, the pattern of which is defined by one of the circuit masks, a programming mask. When this type of mask-programmed, read-only-memory is used for code circuit updating, an updating of one pattern layer will result in a desired pattern change in the mask corresponding to said pattern layer and also in a pattern change (programming) on the programming mask, so as to code the new updating status of the circuit. Thus, both mask which corresponds to the revised or updated pattern layer and the programming mask are renewed before new circuits incorporating the new updating status can be produced.